1. Field of the Invention
This invention relates generally to the liquid phase oxidation of an alkyl aromatic in a solvent with an oxygen-containing gas under an elevated temperature and pressure and in the presence of an oxidation catalyst comprising cobalt, manganese and bromine components, and more particularly concerns a method for effecting this oxidation process using solid cobalt and manganese oxalates as catalyst components.
2. Description of the Prior Art
The liquid phase oxidations of an alkyl aromatic in an acetic acid medium with an oxygen-containing gas are frequently performed in the presence of an oxidation catalyst comprising cobalt, manganese and bromine components. The failure to recover and re-use the cobalt and manganese components of the catalyst system after the liquid phase oxidation represents a significant economic loss and raises a potential environmental problem of disposal. Even when the mother liquor resulting after separation of the aromatic carboxylic acid product and containing dissolved catalyst is recycled to the aforesaid liquid phase oxidation, since undesirable organic by-products accumulate in the mother liquor, generally a portion of the separated mother liquor is not recycled but is discharged as residue. The cobalt and manganese components of this unrecycled portion of the mother liquor are therefore also discharged. Because the recovery of the cobalt and manganese components of the catalyst after completion of the oxidation represents an important factor for the economic operation of such a process, recovery of the cobalt and/or manganese components of the catalyst system by their precipitation as solid oxalates from the organic media has been proposed. For example, Japanese Kokai Pat. No. 51/97592, published British Patent Application No. 2,114,130 and U.S. Pat. Nos. 3,840,469 and 4,246,185 disclose such methods. However, processes to solubilize the cobalt and manganese oxalates and then to use the soluble forms of cobalt and manganese as catalyst components in the aforesaid liquid phase oxidations tend to be complex and costly. Thus, if they could be used directly as catalyst components in the aforesaid liquid phase oxidations of alkyl aromatics, the resulting solid cobalt and manganese oxalates represent a convenient, immediately available, economical and highly desirable source of catalyst material. However, heretofore the direct use of solid cobalt oxalate and/or manganese oxalate from any source--and in particular as recycled from an aforesaid oxalate precipitation technique for recovery of the cobalt and manganese catalyst components--as catalyst components in the aforesaid liquid phase oxidations of alkyl aromatics has not been proposed. For example, U.S. Pat. No. 3,840,469 teaches that the precipitated cobalt oxalate be redissolved as cobalt acetate in an acetic acid solution which could then be used in the liquid phase oxidation.